1. Field of the Invention
The present invention relates to a combined semiconductor device useful in, for example, an optical print head having an array of light-emitting diodes (LEDs).
2. Description of the Related Art
Electrophotographic printers and copiers using an array of LEDs as light sources for image formation typically have an LED unit comprising a plurality of LED array chips and their driver chips. The LED unit is one component of an LED print head that also includes a plurality of rod lenses for focusing the light emitted by the LEDs.
One conventional LED print head, shown in cross section in FIG. 37, comprises a mounting substrate 311 on which are mounted a plurality of LED array chips 312 and driver integrated circuit (IC) chips 313. These chips 312, 313 are electrically interconnected by bonding wires 314 through which driving current is supplied, and the driver IC chips 313 are electrically connected to the mounting substrate 311 by further bonding wires 315 through which control signals and power supply voltages are supplied. A lens holder 317 holds a plurality of rod-shaped lenses 318 in position above the light-emitting parts of the LED array chips 312, the lenses 318 being separated from the LED array chips 312 by a certain distance (h). The lens holder 317 also functions as an enclosure covering the LED array chips 312 and driver IC chips 313. The lenses 318 are held in place by a bead of adhesive material 319. The light emitted by the LED array chips 312 is focused by the lenses 318 onto a photosensitive drum disposed adjacent the LED print head 310. (The photosensitive drum is not shown, and only one LED array chip 312, driver IC chip 313, bonding wire 314, bonding wire 315, and lens 318 are visible.)
Another conventional LED print head (not illustrated) embeds a plurality of optical fibers in a glass base plate, extending through the glass in the thickness direction to form an optical fiber array as described in Japanese Patent No. 3156399. A row of LED array chips are mounted directly on the optical fiber array, embedded in a layer of transparent dielectric resin material. Electrodes formed on the LED array chips or metal projections formed on electrode areas on the LED array chips make contact with a conductive circuit layer which is disposed on the layer of transparent dielectric resin material.
A problem with the conventional LED print head shown in FIG. 37 is that each light-emitting element (LED) of each LED array chip 312 must have an electrode pad to which a bonding wire 314 can be connected. Wire bonding requires comparatively large pads, such as pads one hundred micrometers square (100 μm×100 μm). Referring to FIG. 38, the electrode pads 312c are much larger than the light-emitting elements 312a. The electrode pads 312c must typically be laid out in a staggered arrangement and connected to the light-emitting elements 312a by narrow electrode lines 312b, which take up further space between the front rank of electrode pads 312c. The space occupied by the electrode pads 312c is an impediment to the reduction of the size of the LED array chip 312.
Similarly, as shown in FIG. 39, a number of electrode pads equal to the number of bonding wires 314 must be provided on the surface of each driver IC chip 313 so that it can be connected by the bonding wires 314 to the driven LED array chip 312, and a number of electrode pads equal to the number of bonding wires 315 must be provided on the surface of the driver IC chip 313 so that it can be connected by the bonding wires 315 to the mounting substrate 311. The space occupied by these electrode pads is an impediment to the reduction of the size of the driver IC chip 313.
When an optical fiber array substrate is used, the electrodes or metal projections formed on the LED array chips and the matching electrodes formed in the conductive circuit layer are a similar impediment to the reduction of the sizes of the LED array chips and conductive circuit.
For the reasons described above, as long as electrode pads must be provided, there is a limit to the extent to which the chip size and material cost of the LED array chip 312 and driver IC chip 313 can be reduced. Significant reductions in material cost have been particularly difficult to achieve.
In the fabrication of the LED array chips 312, since the electrode pads 312c occupy a larger area than the light-emitting elements 312a, much more material is required for the non-light-emitting regions than for the light-emitting regions, resulting in extremely poor material usage efficiency. It has been difficult to solve this problem as long as electrode pads are provided on each chip.
To facilitate chip handling during die bonding, to avoid problems such as short-circuits between wires and chips during wire bonding, and to facilitate the formation of wire loops, the thickness of the LED array chips 312 needs to be comparable to that of the driver IC chips 313 (for example, about 300 μm to 350 μm), but this also leads to poor material usage efficiency. A sectional view of the light-emitting part of a typical LED array chip is shown in FIG. 40. A gallium arsenide phosphide (GaAsP) epitaxial layer 324 (referred to below as a GaAsP epi-layer 324) is formed on a gallium arsenide (GaAs) substrate 325, and a zinc diffusion region 321 is formed in the GaAs epi-layer 324. The zinc diffusion region 321 makes electrical contact with an electrode line 322 which is formed on an interlayer dielectric film 323 that covers the GaAs epi-layer 324 except in the area of the zinc diffusion region 321. An underside electrode 326 is formed on the lower surface of the GaAs substrate 325. Light is emitted from a pn junction at the diffusion boundary of the zinc diffusion region 321, but the zinc diffusion region 321 is only about 3 μm to 5 μm deep. In contrast, the GaAs epi-layer 324 is about 50 μm to 100 μm thick in order to reduce the defect density in the neighborhood of the pn junction, and the GaAs substrate 325 is about 250 μm to 300 μm thick in order to facilitate chip handling and match the thickness of the driver IC chip 313.
The 3-μm to 5-μm depth of the zinc diffusion region 321, which functions as the light-emitting region, is only a small fraction of the thickness of 300 μm or more of the LED array chip 312. Matching the thickness of the LED array chip 312 to the thickness of the driver IC chip 313 therefore leads to very inefficient material usage. The GaAs substrate 325 in particular is unrelated to the light-emitting function; it only supports the GaAs epi-layer 324 in which the light-emitting function resides. Nevertheless, the GaAs substrate 325 has to be several hundred micrometers thick to maintain fabrication yields and wire bonding yields, which impedes the reduction of chip thickness, material usage, and accordingly material cost.
Furthermore, the alignment between the light-emitting elements 312a and lenses 318 significantly affects the functioning and characteristics of the LED print head 310. Highly accurate alignment is required to center the lenses 318 on the optic axes of the light-emitting elements 312a in the LED array chip 312 as shown in FIG. 37 and to position the lenses 318 at the correct distance (h) from the LED array chip 312 while keeping them aligned parallel with the light-emitting elements 312a. Maintaining the necessary alignment when the lenses 318 are mounted in the lens holder 317 and the lens holder 317 is attached to the mounting substrate 311 is a difficult process requiring much time and labor.